1. Field of the Invention
The present invention relates to a method of reclaiming silicon wafers that permits of reclamation of silicon wafers once used in manufacturing processes for semiconductor devices, as silicon wafers for testing (used for the purposes of monitoring semiconductor chips manufacturing process, and setting up the operating conditions of semiconductor fabrication machines and equipment). In particular, it relates to a method of reclaiming silicon wafers without Cu contamination.
2. Description of the Related Art
Methods of reclaiming used silicon wafers include, in general, a film removal process for removing films formed on the wafer surface concerned, a polishing process for giving mirror finish to the wafer after the film removal process concerned, a cleaning process for cleaning the polished wafer concerned, and a quality evaluation process for evaluating quality of final products. A process distinctive to reclamation of silicon wafers among the processes is a film removal process. The reason is that silicon wafers after film removal have same state as that of virgin wafers, and accordingly same processes may be adopted as in virgin silicon wafers as a subsequent process (polish process and cleaning process).
Various methods for improving the film removal process and reclaiming silicon wafers have been proposed. For example, Referential Patent 1 (U.S. Pat. No. 5,855,735 specification) discloses a reclamation method including a process to remove films while introducing slight micro cracks, using a polishing liquid including abrasive grain and a rotating pad (refer to claims etc.) Moreover, Referential Patent 2 (U.S. Pat. No. 3,923,567 specification) discloses a reclamation method including an etching method using acids as a method of removing metal films, silicon oxide films, and silicon nitrides (refer to claims etc.) Furthermore, Referential Patent 3 (Japanese Patent Laid-Open No. 9-17833 official report) discloses a reclamation method in which in advance of film removal, estimation of kinds of films currently formed as a film on a silicon wafer concerned is performed based on a measurement of infrared absorption spectrum of used silicon wafers, and subsequently, etching suitable for a type of the film is applied to remove the film (refer to claims etc.).
Since removing of films by the methods may reduce concentration of metals that exist in a surface of films, these methods are effective in reclaiming used silicon wafers having films including main metals (Al, Ti, W, etc.) used during manufacturing processes of semiconductor chips. That is, in reclamation processes of silicon wafers, used silicon wafers are processed having films including various metals, such as metal films, metal silicide films, metal oxide films, and metal nitride films formed thereon. Although metals, such as Al, Ti, and W, might probably deposit again onto a surface of silicon wafers during reclamation in those methods, it was not estimated that the metals might be penetrated inside, and therefore only use of the film removal method enabled reduction of a surface metal concentration.
However, instead of Al conventionally widely used, in many cases, Cu as a wiring material is increasingly used. The reason is that Cu has more excellent electrical conductivity compared with Al, has a high electromigration resistance, and is believed to be more suitable as a wiring material. However, there has been a problem that a very larger diffusion factor in silicon of Cu as compared with other transition metals induces not only deposition of Cu on a silicon wafer surface but penetration inside the silicon wafer during reclamation processes, leading to easy diffusion thereof. Moreover, contamination by Cu spreads in each step of reclamation processes, and concerns are rising that a chain of contamination of furthermore contaminating other products. And removing of Cu penetrated inside is difficult even by the film removal methods mentioned above.
Then, in view of reclaiming silicon wafers with a copper film deposited thereon, Referential Patent 4 (Japanese Patent Laid-Open No. 2002-158207 official report) discloses a method for solving and removing the copper film with a specific etching chemical solution (refer to claims etc.) However, this method is proposed for the purpose of removal of a copper film, and prevention of deposition of Cu on a surface of the silicon wafer, and cannot remove Cu penetrated inside of the silicon wafer. Accordingly, in the case where a silicon wafer reclaimed by this method is used as a test wafer, a particular film (a copper diffusion barrier film or a SiO2 film) is formed on a wafer surface ([0009]).
Besides, Referential Patent 5 (Japanese Patent Laid-Open No. 2000-164558 official report) discloses a method of reclaiming silicon wafers with metal films in which an oxide film is formed in a under layer of a metal film of a test wafer, all metal films and a part of minimum oxide films are removed by chemical etching with alkaline solution or acidic solution, and further metal contaminants on a surface of the oxide film is removed by chemical etching with acidic solution (refer to the claim etc.) However, the method forms a silicon oxide film as a base layer in order to avoid penetration of metals inside of the silicon wafer, such as Cu, and has no intention of removal of Cu penetrated inside of the silicon wafer.
Thus, although conventional methods of reclamation of silicon wafers may remove Cu deposited to a surface of silicon wafers, they may not remove Cu penetrated inside of the silicon wafers.
Although it is not a technique disclosed as a method of reclaiming silicon wafers on the other hand, Referential Patent 6 (Japanese Patent Laid-Open No. 9-64113 official report) discloses a method of detection having a process of heating a semiconductor substrate at temperatures of no more than 600° C., and a process of measuring a surface of the semiconductor substrate for a concentration of Cu, as a nondestructive method for detecting a concentration of Cu diffused inside of a silicon wafer (refer to claims etc.) This method utilizes a phenomenon in which heating of a semiconductor substrate diffuses Cu incorporated inside of the silicon wafer toward a surface thereof, and describes that heating at temperatures of no less than 600° C. diffuses Cu inside of the silicon wafer to gather Cu to a surface of the substrate ([0009]). And it indicates conditions of: at 500° C. for 15 minutes for P-type silicon wafers, and at 500° C. for 2 hours for N-type silicon wafers as a detailed a heated condition ([0012]). However, since heating at such high temperatures impairs quality of silicon wafers, as mentioned below, it is unsuitable to apply the method for a method of reclaiming used silicon wafers.
Moreover, Referential Patent 7 (Japanese Patent Laid-Open No. 2002-270568 official report) discloses a method for managing a metal concentration on a surface of wafers by controlling to limit a concentration of metals (Fe or Cu) in an etching solution and a cleaning liquid that directly contact wafers to no more than a predetermined value in manufacturing semiconductor wafers (claims etc.) This method, however, only control a metal concentration on a surface of wafers in order to avoid depositing of metals, such as Cu, to the surface of the wafers surface ([0005]), and has no intention at all about removing metals penetrated inside of the wafers.